High-efficiency dysprosium-ion extraction enabled by a biomimetic nanofluidic channel

Biological ion channels exhibit high selectivity and permeability of ions because of their asymmetrical pore structures and surface chemistries. Here, we demonstrate a biomimetic nanofluidic channel (BNC) with an asymmetrical structure and glycyl-L-proline (GLP) -functionalization for ultrafast, selective, and unidirectional Dy3+ extraction over other lanthanide (Ln3+) ions with very similar electronic configurations. The selective extraction mainly depends on the amplified chemical affinity differences between the Ln3+ ions and GLPs in nanoconfinement. In particular, the conductivities of Ln3+ ions across the BNC even reach up to two orders of magnitude higher than in a bulk solution, and a high Dy3+/Nd3+ selectivity of approximately 60 could be achieved. The designed BNC can effectively extract Dy3+ ions with ultralow concentrations and thereby purify Nd3+ ions to an ultimate content of 99.8 wt.%, which contribute to the recycling of rare earth resources and environmental protection. Theoretical simulations reveal that the BNC preferentially binds to Dy3+ ion due to its highest affinity among Ln3+ ions in nanoconfinement, which attributes to the coupling of ion radius and coordination matching. These findings suggest that BNC-based ion selectivity system provides alternative routes to achieving highly efficient lanthanide separation.

XPS spectra were performed in an ultrahigh-vacuum system with a base pressure of less than 5×10 -10 mbar using an Al Ka source (ESCALAB 250Xi) and a power of 300 W. To confirm the strong interactions between GLP and metal ions, these samples were modified onto the surface of the PI membrane, which could be monitored directly.This method has been widely used for obtaining the information of ions in nanochannels.
Figure 1g shows the K analysis of the etched PI surface (blue lines) and the GLP-modified PI surface (green, orange, and red lines).The considerable decreases of the K peak prove that GLP molecules are grafted successfully, and there are still residual carboxyl groups on the modified PI surface when the GLP concentrations were 25 and 50 mM.In detail, lots of carboxyl groups (COO − ) were obtained on the surfaces of the etched PI nanochannel.Then K + ions were added to combine with COO − where the K signal can be detected by XPS measurement.After that, a small number of GLPs such as 25 and 50 mM were employed to replace K + ions owing to the strong covalent interactions between GLP and COO − .In that case, the K signal decreased, indicating some GLPs were successfully grafted.In the right amount of GLP, all of K + ions were replaced by GLPs, for example the concentration of GLP is 100 mM.The final coverage of GLP is estimated to be ~100% when GLP concentrations were 100 and 150 mM (Supplementary Fig. 3), and the average density of carboxylate groups on the etched PI surface has been estimated to be ~1.5 groups nm -2 .

Scanning electron microscopy (SEM).
Field-emission SEM (Hitachi S-4800 scanning electron microscope with an accelerating voltage of 10 kV coupled with second electron imaging was performed to observed the structural details of the nanochannel.In order to obtain the high-quality image information, the Au nanolayer was deposited onto the surface of the nanochannel by ion sputtering with an Au target (99.999%)using an ion sputtering system (SBC-12, KYKY Technology Development Ltd.) in a vacuum (>4.0 MPa) for 60 seconds.

Inductively coupled plasma mass spectrometry (ICP-MS).
Inductively coupled plasma mass spectrometry (ICP-MS) was employed to determine metalion concentrations.Every sample was tested at least three times to observe the average values.In this work, uncertainty originates mainly from two routes: experiments (Uncertainty Type A, μA) and measurements (Uncertainty Type B, μB).For accuracy, each experiment was repeated at least three times, and the μA was calculated by equation: where n is the experimental repetition time, xi is the result of experiment i, and ̅ is the average result of n experiments which can be calculated by the equation: For ITC measurement, the analytical precision of the instrument (TA NANO) was better than 5% relative standard deviation, and the Type B uncertainty (μB-ITC) of an experiment (x) was calculated to be: √3 Additionally, the qualities of various chemicals were weighed using an analytical balance (Mettler Toledo) with the analytical precision of ±0.1 mg and thus, the uncertainty was calculated to be: The pH calibration was performed using FE30 (Mettler Toledo) with the analytical precision of ±0.01, and the corresponding uncertainty was calculated to be: 0.01 √3 Finally, the combined standard uncertainty (μ) was calculated by equation: The diameter of the tip can be found to be ~10 nm, which is described by the equation (Supplementary Equation 1):

Supplementary Figures
where κc is the specific conductivity in 1 M KCl solution at 298 K, that is, 0.11173 Ω −1 cm −1 .I, U, l, and D refer to the ionic current through the nanochannel, the voltage, the length of the nanochannel, and the diameter of the base.In principle, the practical diameter of the modified nanochannel can be obtained according to the tip diameter of the unmodified nanochannel, which is equal to the unmodified diameter minus the two lengths of the modified GLP layer.
In case of a GLP monolayer, the size of the modified nanochannel can be calculated to be approximate 8.5 nm. is similar to that of Dy 3+ , while the transport rate of Er 3+ shows a decline.Furthermore, the transport rate of Tm 3+ significantly drops, approaching that of Yb 3+ .Error bars give the standard deviation from three independent tests.

Supplementary Tables
Supplementary Table 1.The differences of Ln 3+ with different coordination numbers on their ion radii.

1. 5 .
X-ray diffraction (XRD) measurements.X-ray diffraction (XRD) measurements were accomplished on Micromeritics Tristar II 3020 with a PANalytical B.V. Empyrean powder diffractometer using Cu-Kα radiation at 40 kV and 40 mA over a range of 2θ = 4.0° up to 40.0° with a step size of 0.02° and 2s per step.1.6.Neodymium-dysprosium solutions.Neodymium (Nd) is an important component of sintered neodymium magnets.Dysprosium (Dy) are also the key components of this material, which increase its intrinsic coercivity.Highperformance Nd-magnets include up to 9% Dy by total magnet weight.The U.S. Department of Energy has categorized dysprosium and neodymium as critical materials because of their supply problems and importance to technology.In this work, we prepared high-concentration Nd solutions with low-concentration Dy (Nd/Dy = 32.3:1,wt.%).1.7.Uncertainty analysis.
. Fabrication of biomimetic nanofluidic channel.The process was divided into three steps, including 1. Irradiation of the PI membrane using UV light; 2. Iontrack etching using NaClO solutions under an applied bias to obtain the single nanochannel; and 3. Modification of GLP into PI nanochannel.In detail, (a) Two faces of polyimide (PI) membrane were first irradiated with an ultraviolet (UV) light for 1 h.(b) The single conical nanochannel was obtained by using the well-developed etching technique at 333 K and a bias of +1 V.The etching process was stopped at a desired current value corresponding to a certain tip diameter.For more details, the PI membrane was clamped between two polytetrafluoroethylene (PTFE) compartments, of which one cell, facing the base of the conical nanochannel, was filled with ~14 wt.% NaClO solution as the etching solutions, while the other cell was filled with 1 M KI solution in order to neutralize the etchant as soon as the channel opened.(c) The carboxyl group surface was activated by soaking in EDC/NHSS aqueous solution for 1 h at 298 K without light.Once finished, the PI membrane was immersed into GLP solutions with different concentrations, such as, 25, 50, 100, and 150 mM.Note that the tip of channel was in contact with GLP solutions.Resultant membrane was washed for several times using DI water.(d) The as-prepared BNC−GLP was clamped between two compartments, which was monitored by a Keithley 6487 picoammeter using the Ag/AgCl electrodes.Supplementary Figure 2. SEM images of nanochannel.As a typical example of showing the dimension of the conical channel, we can calculate the diameter of the base to be 750 nm.

Figure 4 .
Stability of GLP in different conditions for a long-term use.GLP (R)showed the excellent long-term stability in D2O (black line), D2O/DCl (pH=4, red line), and D2O/DCl for one week (pH=4, purple line).Compared with the peaks of GLP in D2O, the corresponding peaks of D2O/DCl exhibited the few shifts, indicating GLP could be stably used.Supplementary Figure5.Effect of BNC on ion transport determined by the changes of ion current.For a clear comparison, we measured the ion current of the unmodified PI nanochannel (red spots/lines) and the modified PI nanochannel (black spots/lines), that is BNC−GLP100, respectively.As shown in Supplementary Figure5, the ion currents of La 3+ (a), Nd 3+ (b), and Eu 3+ (c) ions through the unmodified PI nanochannel are higher than those through the modified nanochannel, which suggesting that the BNC−GLP100 suppresses their transport due to the weaker interactions between ions and surface of nanochannel.On the contrary, the ion currents of Tb 3+ (d), Dy 3+ (e), and Yb 3+ (f) ions through the unmodified PI nanochannel are lower than those through the modified nanochannel, which suggesting that the BNC−GLP100 enhances their transport due to the stronger interactions between ions and surface of nanochannel.Error bars give the standard deviation from three independent tests.resonance shifts for different REEs in D2O (4.71 ppm), with TMS as an internal standard set at 0 ppm.The addition of REEs led to different shifts with respect to the reference peak (GLP, black line), with the extent of the shift indicating the corresponding interactions.The concentration of GLP and REEs was respectively set to 2 mM and 0.2 mM in D2O.Inset shows the structure of GLP with H signals of −CH2 at 3.84 and 4.13 ppm (blue spots) and −CH at 4.15 ppm (pink spots). 1 H chemical shifts were displayed in different colors according to specific species.Supplementary Figure 20.Ion concentration of Dy 3+ and Nd 3+ in the binary solution.The selectivity of Dy 3+ /Nd 3+ was calculated to be 58, according to the ratio of ion concentrations.In the binary solution, the competitive ion transport is far lower than that in the mixture of six REE solutions.Therefore, the selectivity in the binary solution was enhanced.Error bars give the standard deviation from three independent tests.Supplementary Figure 23.Transport rates of other heavy Ln 3+ ions.The transport rate of Ho 3+

Table 2 .
The Ln series divided into light REEs (LREEs) and heavy REEs (HREEs).These values of Tb 3+ are missing, which are presented here using the estimated results according to the lanthanide contraction effect.aThevaluewasobtained by calculating the average value of 0.602 and 0.582.bThevaluewasobtained by calculating the average value of 203.4 and 196.8.cThe value was obtained by calculating the average value of 67.8 and 65.6.Supplementary Table4.Structure of electronic shell of Ln 3+ ions.

Table 7 .
Atomistic coordinates for the mode of the La 3+ -GLP compound optimized by using the PBE0 method.

Table 8 .
Atomistic coordinates for the mode of the Nd 3+ -GLP compound optimized by using the PBE0 method.

Table 9 .
Atomistic coordinates for the mode of the Eu 3+ -GLP compound optimized by using the PBE0 method.

Table 11 .
Atomistic coordinates for the mode of the Dy 3+ -GLP compound optimized by using the PBE0 method.

Table 12 .
Atomistic coordinates for the mode of the Yb 3+ -GLP compound optimized by using the PBE0 method.